and Vegetable Colouring -matters by the Spectrum Microscope. 161 



Hypochlorite of Soda. — This reagent, with or without the addition 

 of citric acid, is sometimes useful, as, for instance, in detecting the 

 adulteration of rhubarb with turmeric ; but generally its action is 

 too powerful and too uniform. 



Permanganate of Potash. — This also usually acts too powerfully 

 on colouring-matters. The excess can easily be removed by sulphite 

 of soda, which makes an alkaline solution pale yellow, but an acid 

 solution quite colourless. 



18. Grouping of Colours. 

 Having now considered some of the chief peculiarities of the most 

 useful reagents, I proceed to describe what appears to me to be the 

 most convenient method of dividing colouring-matters into groups 

 and subgroups, so as to enable us to ascertain the nature of any par- 

 ticular substance under examination. The number of distinct co- 

 loured compounds met within different plants is so great, that some 

 such classification is imperative. In the first place, we cannot do 

 better than divide them according as they are soluble in water or 

 alcohol. This may be looked upon as a chemical division, and is very 

 useful in practice. Thus — 



Soluble in water and not precipitated by alcohol .... Division 1. 



Soluble in water and precipitated by alcohol „ 2. 



Insoluble in water but soluble in alcohol „ 3. 



Insoluble both in water and alcohol „ 4. 



Of course cases occur which cannot be unhesitatingly classed with 

 any one of these ; but they often form good practical divisions, and 

 necessarily modify the methods requisite for further examination. 



19. Method and Order of Experiment. 



If a colour belongs to division 1, a small quantity, sufficient for 

 three or four experiments, should be exposed to the vapour of am- 

 monia in a watch-glass, until there is certainly no longer any free 

 acid, and then gently evaporated, so that all excess of ammonia may 

 be lost. If not thus made neutral we might be entirely misled ; for 

 some pink colours are blues reddened by an acid. A small quantity 

 should then be dissolved in water in one of the small experiment- 

 tubes and the spectrum observed. If too little colour has been added 

 to give the characteristic spectrum, more should be introduced ; but 

 if any part is entirely absorbed, the cell should be turned sideways, 

 in order to see whetner or not some narrow absorption-band occurs 

 there ; and then it may be desirable to remove some of the solution, 

 and fill up the cell with water. As a general rule, so much colour 

 should be added as j make the darkest part of the spectrum de- 

 cidedly shaded but yet not so black as to hide any narrow bands ; 

 and if any occur, the solution should be made of such a strength as 

 to show them to the greatest advantage. This can easily be done, 

 after a little practice, and is made much easier by being able to turn 

 the tubes sideways. Having noted the spectrum of the neutral so- 

 lution, a very small quantity of ammonia should be added, and then 

 a decided excess, the spectra being examined to see if there be any 



Phil. Mag. S. 4. Vol. 34 No. 228. Aug. 1867. M 



